Recently a social demand for an environment-friendly vehicle has increased so that development is in progress of a vehicle that is not driven by an internal-combustion engine that mainly uses conventional gasoline or light oil, but a so-called hybrid electric vehicle that is driven by a combination of the internal-combustion engine and an electric motor or an electric vehicle that is driven by the electric motor. Already some of these vehicles have been put into practical use and have been on sale.
For the hybrid electric vehicle and the electric vehicle, a secondary battery that drives a motor and that can be charged and discharged is essential. But many of the conventional secondary batteries, which are represented by a lithium battery, use a liquid electrolyte. So, there are problems, such as an oil leak.
Also, although the lithium ion battery has a record in that it has often been used as an electric source for a portable device such as a laptop computer, a mobile phone, etc., it has often been reported that the lithium ion battery has caused a fire or explosion. Particularly, a secondary battery that is mounted on a vehicle is required to operate under the conditions that are harsher than those to which the secondary battery that is mounted on these portable devices is exposed. Also, no less important is that since a secondary battery that is mounted on a vehicle needs greater energy capacity, safety measure should be considered.
To meet these social demands, development of the all-solid-state battery, of which all the main components, including an electrolyte, consist of solid material, has been in progress. As the all-solid-state battery has an electrolyte that is not liquid, the chance that it will cause an oil leak, fire, or explosion is greatly reduced. Although an all-solid-state lithium secondary battery can be charged and discharged to and from a high voltage such as 3-5 volts, it has a high level of safety. This is because it adopts a non-combustible solid electrolyte (for example, see Patent Documents 1 and 2).
The technology as disclosed in Patent Documents 1 and 2 relates to a method of manufacturing involving a high cost where the method uses a vacuum vapor deposition method, laser ablation method, sputtering, etc. Also, as a method of manufacturing a solid electrolyte layer for the all-solid-state battery, a positive electrode active material layer, and a negative electrode active material layer, a method of manufacturing by compression molding of raw powder material, a method of manufacturing by screen printing, etc., are proposed.
However, the batteries manufactured by these methods have an insufficient binding force that works between adjacent layers, so that an interface resistance is likely to increase. Also, because of the movement of ions within an electrode active material in the charge- and discharge-processes, a repeated stress is caused between the layers. So, if the binding force that works between the adjacent layers is insufficient, a sufficient interfacial strength cannot be obtained. For this reason, defects such as separation of the layers occurs, which defects are likely to lower the capacity of the batteries or to shorten the lives of the batteries.
To solve these problems an all-solid-state battery is proposed wherein the first electrode and the second electrode are formed by screen printing, etc., and then the first electrode and the second electrode are sintered by a hot-pressing method or a hot-isostatic pressing (HIP) method, thereby lowering the interfacial reaction resistance by increasing the interfacial area between the electrode active material and the solid electrolyte (for example, see Patent Document 3).